Device for detecting the displacement of a writing implement

ABSTRACT

The disclosed device comprises electric conductors (1, 1&#39;; 2&#39;) arranged under the writing surface and associated to a writing implement provided with a permanent magnet of which the power supply is orientated in parallel to its longitudinal axis. Said conductors form elongated meandres-like windings covering the writing surface so that when the implement is displaced in parallel to said surface and tranversely to the meandres of the windings, a sinusoidal voltage is induced at the terminals of the windings provided the instrument is close enough to the writing surface. The pitch of the meandres of the windings is so elected as to induce a sinusoidal voltage having an amplitude higher than a given threshold for a predetermined interval between the writing implement and the writing surface.

The present invention relates to a device for detecting the displacementof a writing implement from a proximity threshold which is determinedrelative to a writing surface, the instrument having, close to itswriting tip, a permanent magnet the magnetisation of which is orientatedso as to be generally parallel to the longitudinal axis of the saidinstrument and symmetrical about this axis, the device comprising,subjacent to the writing surface, at least two electric conductors inseries extending parallel to the writing plane.

French Pat. No. 2 486 271 has already disclosed a device for measuringthe characteristic parameters of the speed of handwriting using coilsdisposed beneath the writing surface in which coils electric voltagesare induced by the magnetic field of a permanent magnet rigid with thewriting implement. If consideration is given to this concept, it will beseen that voltages are induced at the terminals of the coils not onlywhen one is writing, but also consequent upon movements of the writingimplement some distance from the writing surface.

It has been proposed inter alia to use a device of this type to comparethe speed signals of a signature with those of a reference signature. Ina case where this is used, where the comparison is carried outautomatically by comparing digitised signals, the signals which derivefrom movements of the writing implement preceding the signature areparasitic signals which must be eliminated. The distinction betweenparasitic signals and useful signals presents an awkward problem, theaccuracy of which will to a large extent ensure successful comparison ofsignals.

European Pat. No. 0 030 224 discloses a solution to this problem byassociating a piezo-electric detector with the writing surface, in orderto set the start of the comparison at the moment the writing implementtouches the writing surface. In practice, it has been shown that thissystem is frequently subject to malfunctioning in that it is sensitiveto parasitic noise.

The aim of the invention is to obviate, at least in part, thedisadvantages set out above.

To this end, the subject of the invention is a device for detecting thedisplacement of a writing implement according to claim 1.

The advantage of this device over other solutions previously proposedlies essentially in that only the magnetic field emanating from thepermanent magnet rigid with the writing implement can induce acharacteristic signal of the fixed proximity threshold. Consequently,false starts to the signature comparison process for example, caused bya variety of parasitic vibrations transmitted to the writing surface,are avoided. As a result, no other special precautions are necessary fordetermining the start of recording in order to proceed with thecomparison of a signature with a reference signature. This feature is ofgreat importance in a system intended for use by the general public, aswould be the case if this method of comparing signatures were to be usedfor credit cards in order to verify that the holder of a given card isin fact the legal cardholder.

The accompanying drawings show diagrammatically and by way of example anembodiment of the device which is the subject of the present invention.

FIG. 1 is an exploded perspective view of this embodiment.

FIG. 2 is an electronic circuit diagram.

FIGS. 3a to 3c are diagrams of the processing of the signal by thecircuit in FIG. 2.

FIG. 1 shows four electric conductors 1, 2 and 1', 2' each forming aseries of rectangular loops 3, respectively 4, 3' or 4', adjacent ontheir long sides and open on the short side, the opening of the adjacentloops being located at the respective opposite ends of these loops.Having formed the loops 3, 4 respectively, the conductors 1 and 2 form asecond series of rectangular loops 5, and 6 respectively, which areadjacent on their long sides, and open on their short sides, theopenings of the adjacent loops being located at the respective oppositeends of these loops. The loops 5 or 6 respectively overlap the loops 3and 4 respectively, but with their open ends adjacent the respectiveclosed sides of the loops 3 and 4 respectively. The conductors 1', 2'likewise form loops 5' or 6' respectively which are arranged identicallyin respect of the loops 3' and 4' in the same way as the loops 5 and 6in respect of the loops 3 and 4.

In this example, the width of the loops 3 to 6 and 3' to 6' is 1 cm,magnet A secured to shaft B of the writing implement is comprised of acylinder made of samarium cobalt 7 mm in diameter by 10 mm in length,and of which the end closest the tip of the writing implement is set 5mm back therefrom. With a magnet of this type, when the tip of thewriting implement is 5 mm from the writing surface, i.e. when the end ofthe the magnet closest this surface is 1 cm away, the diameter of themagnetic flux cutting the writing surface is 1 cm.

Since the maximum flux variation in a loop, generating a maximum e.m.f.,occurs when the longitudinal axis of the magnet coincides with that ofthe writing implement, and is displaced perpendicular to one of thelongitudinal windings of a loop, and the width of a loop issubstantially equal to that of the flux occurring when the tip of thewriting implement is 5 mm from the writing surface, the writinginstrument has to move substantially parallel to this surface, withmovement components transverse to the loops 3, 3' and 5, 5' and 4, 4'and 6, 6' respectively of at least 1 cm to induce the maximum e.m.f. Toreduce this displacement length to 5 mm between two maximum e.m.f. andincrease the sensitivity of the device, the two conductors 1 and 1' andthe two conductors 2 and 2' connected in series are superimposed, beingstaggered by a half a pitch, so that the displacement of the writingimplement generates a maximum e.m.f. as soon as it is displaced with amovement component transverse to the loops of the conductors 1 and 1' or2 and 2' by at least 5 mm lengthwise and at most 5 mm from this surface.

The major axes of the rectangular loops 3, 3'0 and 5, 5' cut the majoraxes of the loops 4, 4' and 6, 6' at right angles. This arrangementmeans that the four superimposed conductors detect the movementcomponents along two rectangular axes.

The electronic diagram of the processing of signals derived from thecoils 3 to 6 and 3' to 6' comprises a signal amplifier 7, a rectifier 8and FIG. 3c shows the setting of the threshold and the generation ofcontrol pulses each time the detected, amplified and rectified signalexceeds the fixed threshold.

The proposed solution is extremely simple, as the conductors 1, 2 and1', 2' can be made using printed circuit technology. This solution isreliable in that the control pulse can only be generated by the writingimplement associated with a permanent magnet being displaced at a setdistance from the writing surface. Furthermore, this solution is fullycompatible with the process of detecting the speed of the writingimplement by inducing voltages proportional to this speed at theterminals of the induction coils, as described in the above-mentioneddocuments.

I claim:
 1. A device for detecting the displacement of a writingimplement from a proximity threshold determined relative to a writingsurface, this instrument having close to its writing tip a permanentmagnet of which the magnetisation is orientated so as to be generallyparallel to the longitudinal axis of the said instrument andsymmetrically about this axis, the device comprising, subjacent to thewriting surface, at least two electric conductors in series extendingparallel to the writing plane, characterised in that each of theconductors forms a series of extended parallel loops open alternately attheir opposite ends, the major axes of the loops of one conductor beingperpendicular to the major axes of the loops of the other conductor, thewidth of the loops being selected so as to be substantially equal to thediameter of the flux of the said permanent magnet when the said writingimplement is, at the most, a given distance from the said writingsurface, so that the displacement of this instrument induces at theterminals of these conductors periodic voltage variations which aregreater than a given threshold, a detector of the said threshold beingdesigned to emit a command signal as soon as the periodic voltageexceeds this threshold.
 2. A device accoding to claim 1, characterisedin that each conductor forms a second series of extended parallel loopsopen alternately at their opposite ends and superimposed on the loops ofthe first series with their respective open ends superimposed on theclosed sides of the loops of the first series.
 3. A device according toclaim 1, characteriesed in that another electric conductor formed in anidentical manner to the first conductors is superimposed on each of thetwo first conductors with their loops extended parallel respectively tothose of each of the said first conductors, but staggered by half apitch of the loop width relative to the first respective conductors.